Multiuser Detection Techniques for Hierarchical Cell Structures in CDMA Cellular Systems

Abstract

CDMA-only microcell/macrocell cellular systems have traditionally suffered from the near-far problem in the microcells located near the macrocell boundary. Strong macrocell interference in these microcells disables conventional single-user detectors. The solutions currently adopted in the literature include bandwidth-consuming frequency separation between macrocell and microcell tiers and complex hybrid architectures. In this work we present the techniques for multiuser receiver design that address this problem. The preprocessing stage, called the projection stage, which projects the received signal onto the space orthogonal to the subspace spanned by macrocell users' signature sequences, is introduced. It is followed by the stage employing conventional multiuser detection (MUD) techniques. This class of detectors, called multiuser projection detectors (MPD), provides efficient protection against macrocell interference in all microcells, regardless of their location within the macrocell. The protection is optimal if the amplitudes of macrocell interferers are not known by the microcell BS. We analyze four MPDs using different MUD techniques in the detection stage, and perform performance comparison.